So as to suppress carbon dioxide emissions from vehicles, weight saving of a vehicle body using a high-strength steel sheet has been in progress. In addition, in order to secure safety of passengers, besides a soft steel sheet, a high-strength steel sheet is frequently used for the vehicle body. Furthermore, for the weight saving of the vehicle body to progress in the future, it is necessary to increase a strength of the high-strength steel sheet in use further than that of the related art. Accordingly, for example, so as to use the high-strength steel sheet for underbody components, it is necessary to improve local deformability for a buffing process.
However, generally, when the strength of a steel sheet is increasing, formability decreases. Therefore, uniform elongation, that is important for drawing or stretching, decreases. In contrast, Non-Patent Document 1 discloses a method of securing the uniform elongation by making austenite remain in a steel sheet.
Furthermore, Non-Patent Document 1 also discloses a method of controlling a metallographic structure of the steel sheet to improve local ductility that is necessary for a bending, a hole expanding process, or a burring process. In addition, Non-Patent Document 2 discloses that reduction of a difference in hardness between microstructures by controlling inclusions so as to control the microstructures into a single structure is effective for bendability or the hole expanding process.
For coexistence between ductility and strength, Non-Patent Document 3 discloses a technology of obtaining an appropriate fraction of ferrite and bainite. In the technology, a metallographic structure control is performed by a cooling control after hot rolling, precipitates and a transformation structure are to be controlled. However, all of the methods are improving methods of local deformability depending on the structure control (a categorical microstructure control), and thus local deformability is greatly affected by a base structure.
On the other hand, Non-Patent Document 4 discloses a technology of improving a material quality of a hot-rolled steel sheet by increasing a rolling reduction amount in a continuous hot rolling process. This technology is a so-called grain refinement technology. In Non-Patent Document 4, large rolling reduction is performed at a very low temperature in an austenite region to transform non-recrystallized austenite into ferrite. According to this, the grains of ferrite that is a main phase of a product is refined, and thus strength and toughness are increased. However, in the production method disclosed in Non-Patent Document 4, an improvement of local deformability and ductility is not considered.
As described above, for improving local deformability of the high-strength steel sheet, the structure control including inclusions is mainly performed.
In addition, in order to use the high-strength steel sheet as members for vehicles, the balance of strength and ductility is needed. For this requirement, hitherto, a TRIP steel sheet, in which transformation induced plasticity of retained austenite is used, is suggested (for example, refer to Patent Document 1 and Patent Document 2).
However, the TRIP steel has characteristics in which strength and ductility are excellent, but generally, local deformability such as hole expansibility is low. Therefore, it is necessary for local deformability such as hole expansibility to be improved so as to use the TRIP steel, for example, as a high-strength steel sheet of underbody components.